(dataType)invokeJExpression(
                      String expression, 
                      Object[] paramMetadataArray);

(String)invokeJExpression("concat(x1,x2)", new Object [] { "John ", "Smith" });

John Smith

By default, the update strategy for return values is INSERT. To use a different update strategy, you must define the update strategy in the Java code.
If an argument, parameter, or return value is NULL, the value is treated as a null indicator.
For example, if the return value of the invoked expression is NULL and the return data type is a string, the invokeJExpression method returns a string with a value of NULL.
If an input parameter to the invoked expression is a date/time data type, you must pass the parameter as a string and use the TO_DATE function to convert the string to a date/time data type.
For example, use the following argument to pass a date/time value to the invoked expression:
new Object [] { "TO_DATE("01/22/98", "MM/DD/YY")" }
If the invokeJExpression method returns a date/time data type, you must cast the return value to a string.